The present disclosure relates to devices and methods for waveguiding and localizing electromagnetic waves.
When a conductive surface patterned with properly arranged subwavelength features is illuminated by light from a certain range of the electromagnetic spectrum, surface plasmons can be excited. For most metals, this range is in the near-infrared and visible part of the spectrum, where the external field couples to the collective oscillations of the conduction electrons of the metal or a conductor with sufficiently high density of conduction electrons. For simplicity all such conductors with the real part of their permittivity possessing a negative value are simply referred to as conductors herein. At lower frequencies, the near perfect conductivity of the conductor shields the bulk from external radiation. A heuristic approach to circumvent this screening at low frequencies and enhance the penetration of the external fields into the conductor was through the introduction of deep subwavelength corrugations or alternatively by perforating the surface using subwavelength holes into the surface of the conductor. The resulting modes, dubbed spoof surface plasmons, bear resemblance to the visible range plasmonic modes on smooth surfaces where field penetration is significantly more.
It has been shown that such structures are capable of slowing or localizing light when the depth of the corrugations is spatially varied. For example, in U.S. Pat. No. 8,208,191, Gan et al. describe a graded metallic grating structure with a graded depth profile, where the grating consists of grooves having a constant width and a spatially varying depth.